1. Field of the Invention
The invention relates to resin extrusion apparatus and particularly to extrusion heads for producing articles with one or more tubular layers such as parisons, wire coatings, blown films, pipes, pultruded rods, profiles, reinforced sheets, etc.
2. Description of the Prior Art
Parisons, tubular extrusions of plastic resin which are subsequently blow molded to form bottles, are formed in the prior art by various apparatus some including extrusion heads which extrude successive layers of plastic resin onto a mandrel. A typical multilayer parison extrusion head has separate inlets for receiving heated and plasticized resin from individual screw extruders and has separate channels for distributing and expressing the respective plastic resins in successive layers on the mandrel. Each channel includes an annular equalization and distribution chamber surrounding and spaced from the mandrel for receiving the plastic resin from the corresponding inlet. From the equalization chamber the plastic resin is fed through a frustoconical transfer passage downward and inward to a tubular extrusion channel formed around the mandrel. This annular extrusion channel exits through an inwardly or outwardly flaring annular die which includes a conical core member which may be moved longitudinally relative to the outer member of the die to vary the thickness of the wall of the extruded tubular article.
One particularly useful and successful extrusion head for forming tubular articles such as parisons is disclosed in U.S Pat. No. 4,798,526. This head includes one or more individual annular extrusion modules surrounding respective successive portions of a stepped or tapered mandrel to form the annular extrusion channel which receives one or more successively extruded plastic resin layers from the modules. Each module has a pair of members with mating surfaces wherein the equalization and distribution chamber and the frustoconical transfer passage are formed. Having the head formed from individual coaxially spaced extrusion modules enables the head to be readily assembled and disassembled as well as enabling the assembly of an extrusion head with varying numbers of modules so that a module can be used in one assembled head to extrude a single layer tubular article or can be used in a differently assembled head to extrude any layer of a multilayer article. The modules forming an extrusion head can be positioned in any desired order. Furthermore the die modules are separated from each other by annular air spaces. Concentric tubular necks or collars extend from each module into engagement with an adjacent module to define the separation distance or the width of the air spaces between modules. These air spaces prevent heat transfer from a high temperature module to an adjacent low temperature module. Low temperature resins can be degraded if heated to the higher temperature. This patent further discloses polymer inlet pressures at the extrusion head of 4,000 to 6,000 psi (27,000 to 41,000 KPa) for polycarbonate, 2,500 to 4,000 psi (17,000 to 28,000 KPa) for polypropylene, and 2,000 to 3,000 psi (13,000 to 21,000 KPa) for tie resins and barrier resins.
U.S. Pat. Nos. 3,649,143, 4,111,630 and 4,182,603 disclose tubular extrusion dies for blow molding of multi-ply films and having nested frustoconical, hemispherical and cylindrical die members forming polymer distribution chambers wherein spiral and helical grooves are formed on the outer surfaces of inner members. These grooves progress from points near or at the inlets toward the outlets with decreasing depth so that the polymer flow is gradually forced out of the grooves and into the frustoconical, hemispherical or tubular space between die members to evenly distribute the polymer around the chamber. These nested arrangements have several deficiencies such as limiting any temperature differential between the different layers being extruded, requiring larger heads for extruding greater numbers of layers, and having long conical passages from the end of the groove or grooves to the annular outlet.
Generally the prior art tubular extrusion apparatus requires a restrictive frustoconical transfer passage from the distribution chamber to the annular outlet from which the tubular article is extruded. This restrictive passage provides a relatively large pressure drop, i.e., greater than 50% of the total pressure drop from the extrusion head inlet through the distribution region or chamber and the restrictive passage to the outlet, in order to assist in even distribution of the polymer in the distribution chamber. In the absence of the restrictive outlet passage with the relatively large pressure drop, the polymer tends to flow at a greater rate along the shortest path between the inlet to the distribution chamber and the closest region of the annular outlet producing unevenness in the thickness of the tubular article about its circumference.
While the prior art apparatus is generally efficient and successful in the extrusion of tubular articles such as multilayer parisons, blown films, wire coatings, etc., there is room for improvement. The prior art equalization chambers generally have regions where polymer flow is at a standstill or at least significantly reduced. Such reduced flow regions expose a portion of the polymer to heat over a longer period of time to cause polymer degradation. Further, substantially much more polymer flushing is required to flush the old polymer from the relatively dead regions when polymer colors are changed.
Accordingly one object of the present invention is to construct a new and improved polymer extrusion head for tubular extrusions with further improvement in circumferential uniformity in the thickness of the wall of the article being extruded.
Another object of the present invention is to reduce extrusion pressures in tubular extrusion apparatus.
Still another object of the invention is to eliminate relatively low flow rate regions of the polymer distribution path to reduce polymer degradation and to speed polymer changeover operations.
A further object of the invention is to provide improved temperature isolation between extrusion modules in a multi-module extrusion head to enable successive extrusion of plastic layers at greater temperature differentials.